1. Field of the Invention
This invention is concerned with an oil recovery method especially applicable to viscous oil formations, and more particularly is concerned with a multi-phase oil recovery method by means of which the portion of the formation depleted by application of the oil recovery method is expanded vertically and horizontally to achieve more efficient sweep of the formation within the pattern defined by the wells.
2. Background and Prior Art
There are many petroleum-containing formations known to exist throughout the world from which little or no petroleum can be recovered by primary or secondary means because the viscosity of the petroleum is so high that it is essentially immobile at reservoir conditions, and some process must be applied to the formation to decrease the viscosity or otherwise increase the mobility of the petroleum contained in the formation to permit recovery of any significant proportion thereof. The most extreme example are the so-called tar sand or bitumen sand deposits such as those found in the Western United States, Alberta, Canada, Venezuela, and lesser deposits in Europe and Asia. The viscosity of the bituminous petroleum in tar sand deposits ranges upward of several million centipoise at formation temperature, and so substantial viscosity reduction must be accomplished before recovery of petroleum therefrom is feasible.
Viscous oil recovery methods have traditionally involved thermal methods such as steam injection, or in situ combustion, or a method involving injection of a mixture of steam and air for a controlled, low temperature oxidation reaction. While these methods effectively deplete the portion of the formation swept by the fluids, the high viscosity of the formation petroleum and the low viscosity of the injected fluids usually results in the depleted portion of the formation between two or more wells utilized in the heavy oil recovery method representing a relatively small portion of the volume of the pattern defined by the wells utilized in the oil recovery method. Although poor sweep efficiency is a problem experienced in recovery of conventional oils as by water flooding or surfactant flooding, the problem is more severe in viscous oil formations because the sweep efficiency is adversely affected by a high ratio of petroleum viscosity to injected fluid viscosity. Oil recovery processes which in two-dimensional laboratory cells achieve high recovery efficiency, will be very much less successful in field application because the zone depleted by the process is confined to a small portion of the total volume of the formation in the pattern defined by the wells, and the failure to deplete the zone completely occurs in the vertical direction as well as in the horizontal direction.
In view of the foregoing discussion, it can be appreciated that there is a significant need for a method for expanding the zone depleted by viscous oil recovery processes in both the horizontal and vertical direction.